Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/42—Coupling light guides with opto-electronic elements
  • G02B6/4201—Packages, e.g. shape, construction, internal or external details
  • G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/264—Optical coupling means with optical elements between opposed fibre ends which perform a function other than beam splitting
  • G02B6/266—Optical coupling means with optical elements between opposed fibre ends which perform a function other than beam splitting the optical element being an attenuator
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/42—Coupling light guides with opto-electronic elements
  • G02B6/4296—Coupling light guides with opto-electronic elements coupling with sources of high radiant energy, e.g. high power lasers, high temperature light sources
  • G02B2006/4297—Coupling light guides with opto-electronic elements coupling with sources of high radiant energy, e.g. high power lasers, high temperature light sources having protection means, e.g. protecting humans against accidental exposure to harmful laser radiation
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/42—Coupling light guides with opto-electronic elements
  • G02B6/4201—Packages, e.g. shape, construction, internal or external details
  • G02B6/4246—Bidirectionally operating package structures
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/005—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/02—Structural details or components not essential to laser action
  • H01S5/022—Mountings; Housings
  • H01S5/02208—Mountings; Housings characterised by the shape of the housings
  • H01S5/02212—Can-type, e.g. TO-CAN housings with emission along or parallel to symmetry axis
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/02—Structural details or components not essential to laser action
  • H01S5/022—Mountings; Housings
  • H01S5/0225—Out-coupling of light
  • H01S5/02251—Out-coupling of light using optical fibres

Definitions

• the present invention relates generally to the field of optical transceivers. More particularly, embodiments of the present invention relate to eye safety requirements for optical signals transmitted from optical transceivers.
• Related Technology Laser signals are widely employed in a variety of different technologies and applications. For example, lasers have been widely used in military contexts as range finders, as target designators, and in guidance systems. Lasers are also widely incorporated into communication systems for high-speed data transfer. The practical uses of lasers, as well as the physical properties of different lasers, vary greatly. While some lasers emit relatively low power signals, other lasers may emit signals of much higher power.
• Class 1 eye safety requirements provide guidelines for safely transmitting laser signals in environments where unprotected eyes may be exposed to those laser signals.
• Class 1 eye safety limits incorporate limits on laser power and exposure time. Thus, the power of a laser signal may be high for a short period of time, or lower for a long period of time, and still conform to Class 1 eye safety requirements.
• Class 1 eye safety requirements apply to the emission of laser signals in applications such as optical transceivers.
• transceivers For fiber optic transceivers, Class 1 eye safety requirements apply under all conditions, including all reasonable single fault conditions, which are defined as reasonable failures of a single component or connection within a transceiver.
• transceivers are generally designed to ensure eye safety in one of two ways.
• the transceiver may be fundamentally safe because the maximum power the transceiver can emit may be less that the eye safety limit. This is often the case with transceivers incorporating longer wavelength lasers that operate in the range of 1310 - 1550 nm.
• the eye safety limit is ensured by redundant electrical circuits that monitor either the laser current, or, more directly, monitor the laser output power through a monitor photodiode. While eye safety systems based on electrical circuitry are useful for keeping the power of laser signals within the eye safety limits, such eye safety systems may become complex and can increase cost, complicate production, and affect performance of optical transceivers.
• Eye safety systems based on electrical circuitry include redundancies to ensure that the optical transceiver will continue to function in the event of the failure of a single electrical component or connection within the electrical circuitry used in the transceiver.
• These electrical circuitry systems generally serve to cut off the bias current to the laser when a fault is detected, and therefore often consist of two transistors in series with the laser element.
• the series components can reduce the electrical headroom within transceivers, thereby limiting transceiver performance, configuring transistors in series within transceivers may be impractical and inefficient.
• Another example of redundant circuitry used to detect or compensate for single point failures involves the use of monitor photodiodes.
• the output of the monitor photodiode is monitored, and when the output exceeds a preset level, the laser bias current is restricted.
• failure of either the monitor photodiode or the connection to the monitor photodiode must be detected because many systems use the monitor photodiode in a feedback loop to maintain the optical output power in a desired range. If the monitor photodiode, or the connection to the monitor photodiode, fails, the feedback loop will tend to drive the bias current to the maximum level, which in many systems would cause the output power level to exceed eye safety limits.
• embodiments of the present invention which are generally directed to incorporation of an optical limiting material into a transmitter optical subassembly (TOSA) in order to control the optical signal output power and facilitate conformance with eye safety requirements.
• TOSA transmitter optical subassembly
• One such TOSA includes an optical emitter and a fiber receptacle that receives an optical fiber.
• an optical limiting material is positioned between the optical emitter and the fiber receptacle.
• Figure 1 is a cutaway view of an exemplary optical transceiver module wherein optical limiting materials and elements are employed
• Figure 2 is a cutaway view of an embodiment of a TOSA with an optical limiting material bonded directly to the emitting surface of the optical emitter
• Figure 3 is a graph of the output power of an optical signal transmitted from an optical limiting material as a function of the input power of an optical signal entering the optical limiting material
• Figure 4 is a flow diagram showing a process for limiting the power of an optical signal transmitted in a TOSA.
• Figure 5 is a cutaway view of an exemplary ROSA incorporating optical limiting materials
• Figure 6 is a cutaway view of an exemplary ROSA incorporating optical limiting materials on the detecting surface of a detector element
• Figure 7 is a cutaway view of an exemplary ROSA incorporating optical limiting materials in conjunction with a physical contact element
• Figure 8 is a cutaway view of an exemplary ROSA incorporating optical limiting materials and a discrete lens
• Figure 9 is a cutaway view of an exemplary ROSA incorporating optical limiting materials positioned within a lens holder; ,.
• Figure 10 is a graphical representation of output power of an optical signal exiting an optical limiting material as a function of input power of an optical signal entering an optical limiting material; and Figure 11 is a flow diagram showing a method for optically attenuating optical signals.
• Exemplary embodiments of the present invention are concerned with optical transceivers that meet the relevant eye safety requirements by incorporating materials known as optical limiting materials into the design of transmitter optical subassembhes ("TOSA"s) and receiver optical subassemblies ("ROSA”s).
• Optical limiting elements composed of optical limiting materials serve to optically attenuate an optical signal when the optical signal power level exceeds a predetermined threshold.
• a signal is transmitted from the optical emitter, and the signal travels to a fiber receptacle where the optical signal is received by an optical fiber.
• a predetermined limit such as an eye safety limit
• embodiments of the present invention include one or more optical limiting elements between the optical emitter and the optical fiber.
• the optical limiting elements effectively limit the power of the transmitted optical signal by attenuating an optical signal having a power level above a desired threshold.
• optical limiting materials ensure that optical signals transmitted from the TOSA do not exceed eye safety limits and damage unprotected eyes.
• transceiver module 100 includes TOSA 104 and receiver optical subassembly (ROSA) 106 substantially enclosed within housing 102.
• TOSA 104 receives optical fiber 108 while ROSA 106 receives optical fiber 110.
• Each of TOSA 104 and ROSA 106 connects to printed circuit board (PCB) 112 through first electrical contact elements 114 of the PCB 112.
• First electrical contact elements 114 connect to signal traces 116 and circuitry of PCB 112 (not shown), where such circuitry may include a post- amplifier, laser driver, and related circuits, and the signal traces then connect with second electrical contact elements 118.
• Second electrical contact elements 118 connect to external and/or internal electrical components (not shown).
• Optical transceiver module 100 receives and transmits optical signals. When an optical signal is received into optical transceiver module 100, the optical signal enters ROSA 106 through optical fiber 110. After entering ROSA 106, the optical signal is converted from an optical signal to an electrical signal. The electrical signal is transmitted from ROSA 106 to PCB 112 through first electrical contact elements 114. The electrical signal then travels through signal traces 116 and circuitry of PCB 112 to second electrical contact elements 118, and then on to external and/or internal electrical components. In addition to receiving electrical signals from ROSA 106, external and/or' internal electrical components may transmit electrical signals to TOSA 104.
• an electrical signal is transmitted from external components to second electrical contact elements 118 and travels through PCB 112 along signal traces 116 that are connected with circuitry such as a laser driver for example.
• the electrical signal which may be processed by the circuitry, reaches TOSA 104 where the electrical signal is converted to an optical signal.
• the optical signal is then transmitted from TOSA 104 through optical fiber 108.
• TOSA 104 includes housing 120.
• housing 120 is implemented as a single molded plastic element. Housing 120 may also be configured of alternative materials such as glass, for example.
• housing 120 defines fiber receptacle 122 that receives optical fiber 108.
• a fiber stop 124 located at the end of fiber receptacle 122 serves to limit the distance to which optical fiber 108 can be inserted into the housing 120 of TOSA 104.
• the housing 120 of the TOSA 104 is further configured to engage an enclosure 126.
• the enclosure 126 is exemplarily implemented as a TO can and includes a window 128.
• An optical emitter 130 is hermetically sealed by the enclosure 126 and is positioned within the enclosure 126 so that optical signals from the optical emitter 130 pass through the window > 128.
• the optical emitter 128 is a laser, examples of which include, but are not limited to, vertical cavity surface-emitting lasers ("NCSEL"), Fabry-Perot ("FP”) .
• the laser or other optical emitter employed in the TOSA 104 may be selected to generate optical signals at a particular desired wavelength. In one exemplary embodiment, the laser emits optical signals having wavelengths of about 850 nm. In other exemplary embodiments, the laser emit optical signal having wavelengths of about 1310 nm, or. about 1550 nm.
• a lens 132 is positioned within TOSA 104 to focus an optical signal from optical emitter 128 into optical fiber 108.
• an . optical limiting element 134 is positioned within TOSA 104 between optical fiber 108 and optical emitter 128.
• One or more optical limiting element(s) 134 may be positioned at any location, or at multiple locations, between optical fiber 108 and optical emitter 128. In the particular example of Figure 1, the optical limiting element 134 positioned on window 130 of enclosure 126.
• Various materials may be used in the construction of optical limiting elements such as optical limiting element 134. Examples of such material include, but are not limited to, glass, transparent glass gels, polymers, and semiconducting polymers blended with materials having non-linear optical properties such as, for example, fullerenes.
• the optical limiting element may be constructed with a polymer, or a polymer mixed with a dopant.
• the electrical signal is converted to an optical signal which is then emitted from optical emitter 128.
• the optical signal travels from optical emitter 128 through window 130 of enclosure 126 to optical limiting element 134. If the power of the optical signal transmitted to optical limiting element 134 is below a predetermined limit, the optical signal remains substantially unchanged as the optical signal travels through optical limiting element 134. If, however, the power of the optical signal is above a predetermined limit, the optical limiting element 134 optically attenuates the power of the optical signal. Thus, the optical limiting element ensures that the power of the optical signal transmitted from TOSA 104 does not exceed a predetermined limit.
• the predetermined limit corresponds to eye safety requirements.
• the eye safety requirements are Class 1 eye safety requirements.
• Exemplary TOSAs with Optical Limiting Materials Directing attention now to Figure 2, an exemplary TOSA 200 is shown.
• TOSA 200 includes housing 202 which defines fiber receptacle 204 and an associated fiber stop 206;
• An optical fiber 208 is received into fiber receptacle 204 and extends to fiber stop 206.
• the housing 202 also includes , an integral lens 210 and is configured to attach to enclosure 212.
• a window 214 of enclosure 212 is positioned so that an optical emitter 21 hermetically sealed within enclosure.212 is able to transmit optical signals through the window 214 and into the optical fiber 208.
• the TOSA 200 also includes optical limiting element 218 located, in this- exemplary embodiment, on the emitting surface of optical emitter 216.
• the optical limiting element serves to attenuate, if necessary, the power of optical signals from optical emitter 216.
• the optical limiting element 218 shown in Figure 2 is placed directly on the emitting surface of optical emitter 216, in other exemplary embodiments, the optical limiting element may be placed at any position, or at multiple positions, between optical emitter 216 and fiber receptacle 204.
• optical limiting materials may be incorporated into other components of the TOSA 200, such as, for example, the lens 210 and/or the fiber stop 208.
• an alternative embodiment of the TOSA includes a housing, or portion thereof, configured of a material which is doped with an optical limiter compound, such as, for example, a two-photon absorption dye. Any other suitable doping materials may alternatively be employed however.
• the housing is configured so that an optical signal emitted from an optical emitter passes through a portion of the housing before entering the optical fiber. In this exemplary implementation then, no separate optical limiting element is provided, and the optical limiting functionality is implemented by the TOSA housing itself.
• the housing 502 of the ROSA 500 is a single plastic molded element. Housing 502 may be fabricated of any number of different materials including, for example, plastic, glass, or any other optically suitable material. As indicated in Figure 5, the housing 502 of ROSA 500 engages an enclosure 504, exemplarily implemented as a TO-can, within which a detector element 506 is disposed and hermetically sealed. In one embodiment of the invention, detector element 506 is an avalanche photodiode detector (APD).
• APD avalanche photodiode detector
• detector elements include, but are not limited to, a PIN photodiode.
• a lens 508 of the enclosure 504 is configured and positioned to focus an optical signal onto detector element 506.
• the lens 508 is integral with the enclosure 504, but comprises a discrete component in some alternative embodiments.
• the housing 502 defines a fiber receptacle 510 within which an optical fiber 512 is received.
• the fiber receptacle 510 communicates with a fiber stop 514 that serves to limit the extent to which the optical fiber 512 can be inserted into the housing 502 of the ROSA 500: Similar to the exemplary arrangement disclosed in Figure 1, optical limiting material 516 is positioned in ROSA 500 between optical fiber 512 and detector element 506 to attenuate, if necessary, the power of optical signals received into the ROSA 500 from optical fiber 512, so as to prevent damage to the detector element 506.
• the optical limiting element 516 takes the form of a block of material that is attached to the housing 502 so as to be positioned between the fiber stop 514 and the lens 508.
• optical limiting element 516 may be modified as necessary.
• the optical signal is , transmitted from optical fiber 512 through fiber stop 514 and optical limiting element 516. If the power of the optical signal received at the optical limiting element 516 from the optical fiber 512 is below a predetermined limit, the optical signal passes through optical limiting element 516 without attenuation.
• optical limiting element 516 optically attenuates the power of the optical signal to the extent necessary to ensure that the optical power of the optical signal at the detector element 506 is within acceptable limits.
• the detector element 506 detects the optical signal and converts the received optical signal to an electrical signal, m this way, the optical signal that ultimately reaches optical detector 506 has been optically attenuated, if necessary, to a level which is below an optical overload limit, damage threshold, and/or other predetermined limit(s) associated with optical detector 506.
• the ⁇ osition(s) of the optical limiting element(s) within a ROSA can be varied.
• the exemplary ROSA 600 includes a housing 602 configured to engage an enclosure 604.
• a detector element 606 is positioned within the enclosure 604 and is arranged to receive optical signals from a lens 608, where the lens 608 is implemented as an integral portion of the enclosure 604.
• optical limiting element 610 is provided that is positioned directly on the detecting surface of optical detector 606.
• the housing 602 of the ROSA 600 defines a fiber receptacle 612 within which is received an optical fiber 614.
• a fiber stop 616 is defined by the housing 602 at one end of the fiber receptacle 612 so as to limit the depth to which optical fiber 614 can be inserted into the housing 602 of the ROSA 600.
• optical fiber 614 receives an optical signal
• the optical signal travels from optical fiber 614 through fiber stop 616 to lens 608.
• the lens 608 then focuses the optical signal and passes the optical signal to the optical limiting element 610, located on the detecting surface of detector element 606.
• the optical limiting element 610 attenuates, to the extent necessary, the power of the optical signal transmitted to the detector element 606, so as to prevent damage to, or other problems with, the detector element 606.
• the extent to which such attenuation is implemented, if at all, may be defined, for example, by an optical overload limit, or a damage threshold of the detector element 606. More generally, the extent to which optical attenuation is implemented by the optical limiting element 610 can be determined with reference to any of a variety of thresholds and limits.
• a ROSA 700 is disclosed that includes a housing 702 attached to an enclosure 704.
• a detector element 706 is disposed within the enclosure 704 and hermetically sealed therein by the cooperation of the enclosure 704, and a lens 708.
• the lens 708 serves to focus an incoming optical signal onto the detector element 706.
• the housing 702 of ROSA 700 defines a fiber receptacle 710 which receives an optical fiber 712.
• the exemplary embodiment disclosed in Figure 7 further includes a physical contact element 714 positioned to contact the fiber receptacle 710 in order to limit reflection within ROSA 700.
• physical contact element 714 ensures that optical fiber 712 does not extend beyond the end of fiber receptacle 710.
• physical contact element 714 is glass, but embodiments of the invention are not limited to glass and may include plastic or other suitable materials.
• optical limiting element 716 is positioned in ROSA 700 between the physical contact element 714 and the detector element 706, and the optical limiting element is in contact with the physical contact element 714.
• an optical signal is received into ROSA 700 through optical fiber 712.
• the optical signal travels through physical contact element 714 and into the optical limiting element 716. If the power of the optical signal transmitted into the optical limiting element 716 is below a predetermined limit, the optical signal remains substantially unchanged as the optical signal passes through the optical limiting element 716.
• FIG. 8 a cut-away view of an alternative ROSA, denoted generally at 800 and incorporating optical limiting elements and a discrete lens, is shown. As shown in the Figure, a housing 802 of ROSA 800 engages enclosure 804, and a detector element 806 is disposed within the enclosure 804.
• the enclosure 804 includes a window 808 positioned proximate the detector element 806 to allow an optical signal to pass through enclosure 804 to the detector element 806.
• This alternative embodiment differs from other exemplary embodiments in that the optical limiting material 810 is located on the window 808 of enclosure 804, in order to limit the power of optical signals transmitted to the detector element 806.
• the lens arrangement employed in ROSA 800 likewise differs from that of some other embodiments.
• a discrete lens 812 is provided that is held in place by lens holder 814, and positioned within ROSA 800 to focus an optical signal onto detector element 806.
• a physical contact element 816 is interposed between the lens holder 814 and fiber receptacle 818 and, in at least some embodiments, the physical contact element contacts one or both of the lens holder 814 and fiber receptacle 818.
• an optical signal received into ROSA 800 is initially transmitted from optical fiber 820 through physical contact element 816 to lens 812.
• Lens 812 focuses the optical signal, which is then transmitted through optical limiting element 810 located on the window 808 of enclosure 804. If the power of the optical signal transmitted to optical limiting element 810 is below a predetermined limit, the optical signal remains substantially unchanged as the optical signal passes through optical limiting element 8 0.
• Figure 9 shows a cut-away view of a ROSA 900 that includes a housing 902 and enclosure 904 engaged with each other.
• a detector element 906 is provided that is disposed within the enclosure 904.
• the housing 902 defines fiber receptacle 908 within which is disposed an optical fiber 910.
• the ROSA 900 further includes a physical contact element 912 and lens holder 914.
• Optical limiting element 914 is positioned in the lens holder 916 between optical fiber 910 and detector element 906, and the lens holder additionally retains one or more lenses 918, and/or other optical components.
• the optical signal enters ROSA 900 through optical fiber 910.
• the optical signal then travels through physical contact element 912 and optical limiting element 914. If the power of the optical signal transmitted to optical limiting element 914 is below a predetermined limit, the optical signal will remain substantially unchanged as the optical signal travels through optical limiting element 914.
• the optical limiting element 914 optically attenuates the power of the optical signal to ensure that the power of the optical signal detected by detector element 914 is below the predetermined limit.
• the optical signal continues through lens 918, where the optical signal is focused onto detector element 906. The optical signal is then received and converted to an electrical signal by detector element 906.
• the disclosed optical limiting elements are exemplary structural embodiments of a means for optically attenuating the power of an optical signal.
• the scope of the invention is not limited to the exemplary types and arrangements of the exemplary optical limiting materials disclosed herein.
• optical limiting materials are materials having non-linear optical properties, for at least some optical power ranges, whereby transmissivity through the optical limiting material is relatively high for low powered optical signals and transmissivity decreases to a relatively low level if the power of the optical signal exceeds a predetermined upper limit.
• the optical limiting material absorbs, if necessary, a portion of the energy of the optical signal that enters the optical limiting material, h this way, the power of optical signals that exit the optical limiting material is kept at or below a predetermined limit.
• optical limiting materials have characteristic response times for attenuating optical signals that exceed a given power threshold.
• the particular desired response time may vary from one application and/or device to another.
• the response time of a particular optical limiting material is closely tied to standards, such as eye safety requirements, that provide guidelines for the permissible power level of an optical signal, as well as for the permissible maximum time that an eye may be exposed to the optical signal.
• the response time of an optical limiting material is related to the permissible time that an eye can be exposed, without harm, to an optical signal having a certain power.
• the response time varies from one optical limiting material to another and is typically selected with reference to eye safety standards, for example, and the anticipated power of the optical signals that will be involved. Exemplary response times are measured in time lengths anywhere from hundreds of seconds to micro-seconds.
• the response time of the optical limiting material is in the range of about 100 ⁇ s to about 100ms. Because eye safety levels are a function of the total time the eye is exposed to a given power level, time limits for exposure to relatively low powered optical signals may be significantly higher than time limits for exposure to relatively high powered optical signals. In addition, if an optical limiting material is able to respond to a relatively high-powered optical signal relatively quickly, the output power of the optical signal could remain relatively high for a relatively longer period of time without exceeding eye safety limits. Another parameter of optical limiting materials relates to the effect, on the optical limiting materials, of optical signals whose power is above a predetermined limit.
• the response of optical limiting materials to optical signals transmitted by the optical emitter of the TOSA may be reversible or irreversible.
• high powered optical signals are attenuated by both reversible and irreversible optical limiting materials
• the transmissivity of the reversible optical limiting materials returns to relatively high levels when the power of the optical signal decreases below the predetermined power limit, hi contrast, irreversible optical limiting materials are unable to return to high levels of optical signal transmissivity once an optical signal exceeds the power threshold.
• Reversible optical materials can further be divided into at least two categories. Materials in the first category are largely absorptive, and may also be known as two-photon absorption materials. Materials in the second category are largely refractive.
• refractive optical limiting materials are suitable for use in optical systems having a strongly converging or diverging optical beam
• refractive and/or absorptive optical limiting materials are useful in exemplary embodiments of the present invention.
• an optical limiting material is selected for use in exemplary embodiments of the invention based on specific properties of the optical: limiting material,, examples of which include response time, transmissivity and reversibility. Another consideration in the selection of a particular optical limiting material and/or arrangement of. optical limiting material relates to the wavelength(s) of the associated optical signal.
• an optical limiting material having a limiting power in the range of about -3 dBm to about -1.3 dBm could be selected to provide the desired optical attenuation functionality.
• the upper bound of the optical signal power that can be transmitted through such an optical limiting material, and the maximum eye safety limit for optical signal power, is about -2 dBm.
• These power limits refer to the power of the optical signal received at the optical fiber. Of course, such power limits are exemplary only and are not intended to limit the scope of the invention.
• the optical limiting material may be selected for use with optical signals of other wavelengths as well.
• some embodiments of the invention employ lasers that emit signals of 1310 nm, and 1550 nm, respectively. This wavelength information would thus inform the selection of particular optical limiting material(s).
• response times for optical limiting materials selected for use in TOSAs transmitting signals in at 1310 nm or 1550 nm can be relatively longer than response times for optical limiting materials selected for use in TOSAs transmitting signals in the 850 nm range, without exceeding eye safety requirements.
• embodiments of the invention employ a variety of different optical emitters, transmitting at various wavelengths.
• the optical limiting material can be effectively employed with a variety of optical signals generated by devices such as, but not limited to, FP lasers, DFB lasers, and VCSELs.
• optical limiting materials are suited for use with various types of detectors, examples of which include . avalanche photodiodes ("APD"), and P-I-N photodiodes.
• APD avalanche photodiodes
• P-I-N photodiodes P-I-N photodiodes
• the particular optical limiting material to be employed in a given situation is typically selected with respect to the particular wavelength, or range of wavelengths, that the optical limiting material is expected to encounter. Examples of such wavelengths include, but are not limited to, 1310 nm and 1550 nm.
• the foregoing are examples only however and the scope of the invention should not be construed to be limited to any particular device, configuration or operating wavelength(s). It was noted earlier herein that one damage mechanism for detector elements is thermal in nature.
• exemplary optical limiting materials limit the time the detector is exposed to the high powered optical signal to a period significantly shorter than the time period necessary to raise the temperature beyond a critical point.
• the time scale is in the range of microseconds, or about 10 " ⁇ - 10 "3 seconds. Therefore, optical limiting materials incorporated into some embodiments of the invention are characterized by response times of about 10 "6 to about 10 "3 seconds. However, the response times of optical limiting materials used in the present invention will vary and are not limited to any particular times.
• optical limiting materials relate to the effect, on the optical limiting materials, of optical signals whose power is above a predetermined limit.
• the response of optical limiting materials to optical power signals received into the ROSA may be reversible or irreversible.
• high powered optical signals are attenuated by both reversible and irreversible optical limiting materials
• the transmissivity of the reversible optical limiting materials returns to relatively high levels when the power of the optical signal decreases below the predetermined power limit.
• irreversible optical limiting materials are unable to return to high levels of optical signal transmissivity once an optical signal exceeds the power threshold.
• Reversible optical materials can further be divided into at least two categories. Materials in the first category are largely absorptive, and may also be known as two-photon absorption materials. Materials in the second category are largely refractive.
• refractive optical limiting materials are suitable for use in optical systems having a strongly converging or diverging optical beam
• refractive and/or absorptive optical limiting materials are useful in exemplary embodiments of the present invention.
• embodiments of the present invention incorporate optical limiting materials, examples of which include absorptive or refractive materials, into ROSAs to improve the optical overload limit, and to increase the damage threshold for detector elements.
• optical limiting materials with suitable response times and energy absorption helps ensure that the optical power reaching detector element is maintained below the optical overload limit, or other predetermined limit, of the detector element for a large range of optical input powers.
• optical limiting materials into a ROSA can be performed without adverse impact to the overall structure and design of the associated transceiver.
• Various aspects of embodiments of the invention can be modified as necessary to allow the use of particular arrangements and materials and/or to achieve desired effects.
• the optical limiting function can be implemented in a ROSA where the distance between the detector element and the optical fiber is about 1 mm or less.
• the optical limiting material has the mechanical characteristics of glass, which is likely for limiters based on glass doped with appropriate absorbers. For softer materials, a substrate made of glass can be used to support the material.
• some embodiments of the present invention use optical limiting materials that provide little or no optical attenuation effect until the power of the optical signals input to the optical limiting material reaches a level within the range of +3 to +10 dBm, and most typically + 6 dBm, as determined by the damage threshold of the detector element. If the optical limiting material is designed to extend the normal operating range of the receiver, then the threshold at which optical attenuation begins falls within the range of -6 to +3 dBm, where this threshold is chosen based on the saturation level of the detector.
• the properties of the optical limiting material may make it desirable to place the optical limiting material in a portion of the optical path with a suitable optical signal cross section.
• the location in the optical path may be chosen to tune the effective power limit of the ROSA or transceiver.
• the optical limiting material is also chosen to have a response time which is shorter than the time at which the optical detector may become damaged by the power of the optical signal. In general then, various parameters concerning the optical limiting materials may be adjusted as necessary to suit a particular application.
• Such parameters include, but are not limited to, transmissivity, reversibility, response time, range of limiting power, positioning of the optical limiting materials, and energy absorption.
• various parameters concerning the optical limiting materials may be adjusted as necessary to suit a particular application. Examples of such parameters include, but are not limited to, transmissivity, reversibility, response time, range of limiting power, positioning of the optical limiting materials, wavelength of the optical emitter, and energy absorption.
• V. Performance of Optical Limiting Materials With attention now to Figure 3, a graphical representation of output power from the optical limiting material used in a TOSA as a function of the input power to the optical limiting material shows the optical limiting properties of optical limiting materials.
• Optical limiting materials used in exemplary embodiments of the present invention are selected so that the transmissivity of the optical limiting material corresponds with a specified power threshold, as shown in Figure 3 by line 302.
• the power threshold is the damage threshold of the detector element.
• the power threshold is the optical overload limit of the detector element.
• the transmissivity response of the optical limiting material flattens and may approach zero for increases in input optical power beyond the input power threshold.
• the transmissivity of the optical signal returns to a level near 100% when the input power of the optical signal drops below the input power threshold indicated at "B.”
• a graphical representation of output power from the optical limiting material used in a ROSA as a function of the input power. to the optical limiting material shows the optical limiting properties of optical limiting materials.
• Optical limiting materials used in exemplary embodiments of the present invention are selected so that the transmissivity of the optical limiting material corresponds with a specified power threshold, as shown in Figure 10 by line 1002.
• the power threshold is the damage threshold of the detector element.
• the power threshold is the optical overload limit of the detector element.
• the output power is substantially equal to the input power.
• the transmissivity through the optical limiting material is approximately 100%. That is, there is a 1:1 ratio, or unity, between output optical power and input optical power.
• the optical limiting material Attenuates the power of the optical signal, thereby preventing the power of the output optical signal from exceeding the power threshold, notwithstanding any further increase in the input power beyond the input power threshold "B." That is, for at least a predetermined range of input powers, increases in the input power to the optical limiting material beyond point “B” do not result in significant, changes in the output power from the optical limiting material, due to the attenuation properties of the optical limiting material.
• the transmissivity response of the optical limiting material flattens and may approach zero for increases in input optical power beyond the input power threshold.
• FIG. 4 a method for processing an optical signal within a TOSA is shown.
• an electrical signal is converted to an optical signal.
• the optical signal is then emitted, by a laser for example, as shown at stage 404.
• the power of the optical signal is optically attenuated, as shown at stage 406 so that the power of the output optical signal remains below a predefined power limit.
• the attenuated optical signal is transmitted.
• the power of the optical signal transmitted is thereby maintained at a power below the eye safety limit, or at some other predetermined threshold.
• Embodiments of the present invention maintain optical signal output power levels within a desired limit by incorporating optical limiting materials into components such as
• a method 1100 for attenuating the power of an optical signal exceeding a predetermined limit is shown.
• an optical signal is received. If the power of the received optical signal exceeds a predetermined limit, the power is optically attenuated as shown at stage 1104. In this way, no input optical signal having a power over a predetermined limit is received at the detector element.
• the attenuated optical signal is focused. The attenuated optical signal is then detected, as shown at stage 1108.
• the attenuated optical signal is converted to an electrical signal.
• the present invention may be embodied in other specific forms without departing from its spirit or essential characteristics.
• the described embodiments are to be considered in all respects only as illustrative and not restrictive.
• the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within. their scope.

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